The present invention is a chemical and biological sensor based on optical methods of detection.
Needs exist for rapid, sensitive methods of detection and monitoring of chemical and biological warfare materials. Candidate chemical warfare sensors and biological warfare detection instruments should be small, have few or no moving parts, and should be amenable to use in joint chemical and biological detection.
Optical chemical and biological sensors are used in a network of point detectors for a range of applications including monitoring decontamination of Army field structures, detection of chemical and biological agents in chemical treaty verification (see Table I), reconnaissance of battlefield and depot perimeters, demilitarization procedures and monitoring breakthrough times associated with polymeric or other complex structural materials. Selected tasks associated with each application and sensitivities required are presented in Table II.
At present, sensors are available that are capable of meeting the requirements of several applications but no sensor has provided the combined sensitivity and speed of response necessary for each application. Needs exist for field-usable chemical and biological sensors for the detection of vapor and liquid dispersed chemical warfare agents, toxins of biological origin and aerosol-dispersed pathogenic micro-organisms. Existing instrumentation used in identifying chemical warfare agents rely upon ion mobility spectroscopy (IMS) or gas chromatography for detection. The advanced chemical agent detection/alarm system (ACADA) uses ion-mobility spectroscopy to achieve sensitivities to GB/GD on the order of 1.0 mg/m.sup.3 (170 ppb) in 10 seconds and 0.1 mg/m.sup.3 (17 ppb) in 30 seconds. The size and weight characteristics of the ACADA system 0.028 cubic meter (1 cubic foot) in volume and 11.34 kilograms (25 pounds) in weight reduce the applicability of this instrument for distributed sensing or remote sensing applications. Sensors such as the miniCAMS system provide unparalleled sensitivity but require preconcentration times on the order of minutes which are unsuitable for rapid detection of conditions that are immediately dangerous to life and health (IDLH). Other methods under consideration use acoustic or optical/electrochemical methods of detection such as surface acoustic wave (SAW)-based instruments and light addressable potentiometric sensor (LAPS) (Hafeman et aL, 1988). The SAW instrument has demonstrated sensitivities to GB/GD at 0.01 mg/m.sup.3 (1.7 ppb) but requires preconcentration times of from 2 minutes to 14 minutes. Both SAW and LAPS systems have been used in conjunction with immunoassay procedures to detect organophosphorus chemical agents (Rogers et al., 1991; Mulchandani and Bassi, 1995).
Optical methods of detecting organophosphorus-based nerve agent materials have been reviewed by Crompton (1987). One of the best calorimetric methods for detection of organophosphorus halides involves the use of diisonitrosoacetone reagent or the monosodium salt of this material which upon exposure to GA Tabun or GB Sarin at concentrations of micrograms per milliliter produces a magenta color with maximum response within seven minutes. Chemical analysis using 3-aminophthalkydrazide (luminol) with sodium perborate has been shown to be effective in detecting as little as 0.5 microgram of GB Sarin or GA Tabun. The use of polymer-coated optical waveguides in the detection of nerve agents or simulated nerve agents such as dimethyl methylphosphonate (DMMP) has been reported by Giuliani et al. (1986), who identified polymeric materials with an affinity for the nerve agent exhibiting a change in refractive index upon absorption of the nerve agent. Several materials have been found to exhibit an affinity for DMMP. Fluoropolyol, described by Grate and Abraham (1991), was found to have a partition coefficient for vapor phase DMMP between one million and ten million indicating that the concentration of DMMP in the fluoropolyol was up to ten million times that in the vapor phase. Fluoropolyol is strongly acidic, a factor that improves sensitivity to strongly basic vapors such as the organophosphorus compounds.
Fluorescence methods of chemical characterization rely for their operation on the use of light energy of intensity, I.sub.ex, to excite a fluorescent molecule from a ground state E.sub.0 to an excited state E*. Molecules of the fluorophore decay from the excited state either through nonradiative energy transfer, emission of heat or radiative emission of a photon at a wavelength, .lambda..sub.em, longer than that of the excitation source .lambda..sub.ex. At low concentrations of the fluorophore, the intensity of the fluorescence emission, I.sub.em, is directly proportional to the incident light intensity through EQU I.sub.em =Q I.sub.ex (1-exp(-.epsilon.bc)) (1)
where Q is the quantum yield of the fluorophore, .epsilon. is the molar absorptivity, b is the path length and c is the concentration of the fluorophore. The sensitivity of optical sensors to chemical and biological agents can be increased by restricting the excitation energy to a limited volume or area, by increasing the collection efficiency or by increasing the intensity of the excitation source. For this reason, several total internal reflection techniques have been developed to improve solid surface chemical assays (Kronick and Little, 1975; Andrade et al., 1985). Total internal reflection is the phenomenon occurring when light originating in an optically dense region with a refractive index, n.sub.i, impinges upon a boundary separating the dense medium from a less-dense medium with refractive index, n.sub.t. For a small angle between the incident light and the normal to the interface, a certain amount of light is transmitted and the remainder is reflected. As the incidence angle, .theta., exceeds a critical value, .theta..sub.C, given by Snell's law, ##EQU1##
the magnitude of the transmitted beam decreases to zero, and all of the light is internally reflected. For silicon nitride on silica on silicon waveguides, the refractive index of the waveguiding layer of silicon nitride is approximately 1.99 and the refractive index of the silica is 1.46. The critical angle is 47.2.degree.. For ion-exchanged soda lime glass, the refractive index of the base material is 1.512 and the refractive index of the silver ion-exchanged glass is 1.605. If a step-index waveguide is produced, the critical angle is 70.4.degree.. The critical angle is decreased by increasing the refractive index difference between the base material and waveguide. In total internal reflection, the excitation intensity, I, of the evanescent wave varies with distance from the interface in the less-dense medium as the inverse exponential of the distance, x, over a characteristic depth, d, as follows EQU I(x).varies.I.sub.0 e.sup.-x/d (3)
where I.sub.0 is the light intensity at the surface. The penetration As depth, d, of the evanescent field is controlled by the excitation wavelength, .lambda..sub.0, the refractive indices of the media and the angle of incidence, .theta., according to the equation, ##EQU2##
An optical sensor is used to detect chemical warfare agents through detection of changes in fluorescence upon exposure to the agent of a fluorophore immobilized in a solid matrix. The use, as chemical sensors, of fluorophores immobilized in polymeric matrices has been reviewed by Wolfbeis (1985) and Taib and Narayanaswamy (1995), among others.
An array of sensitive non-selective sensors, each having a differential response to the analyte of interest, is used to identify selectively that analyte and provide a quantitative measure of the concentration of the analyte. That principle has been demonstrated by Grate et al. (1993) and Rose-Pehrsson et al. (1988) in the use of an array of surface acoustic wave sensors to detect organophosphorus and organosulfur analytes at low concentrations. Similarly, work has been performed by Freund and Lewis (1995) to characterize odorant materials using the change in the electrical characteristics of an array of poly(pyrrole) conducting polymer-based capacitive probes. Arrays of resistors based on carbon black incorporated in a range of polymeric materials have been shown to be sensitive to parts-per-thousand levels of various solvent vapors. Data reported by researchers at Tufts University (Dickinson et al., 1996; White et al., 1996) demonstrated that an array of cross-reactive fluorescence-based sensors could be used to measure selectively the concentration of organic vapors in an air stream.
Vapor solubility parameters are matched with polymer solvent phase solubility parameters to provide probe materials with large partition coefficients for the vapors of interest. The approach to selecting polymer materials for use in chemical sensors is provided by McGill et al. (1994), and is based on prediction of partition coefficient using the linear salvation energy relationship (LSER). The relationship between the log of the gas-polymer partition coefficient, K.sub.p, and a number of solvation parameters is given by EQU LogK.sub.p =C+rR.sub.2 +s.pi.H.sub.2 +a.alpha.H.sub.2 +b.beta.H.sub.2 +lLogL.sup.16 (5)
where R.sub.2 is the excess molar refraction, a term which models polarizability contributions from n and n electrons, .pi.H.sub.2 is the dipolarity, .alpha.H.sub.2 is the hydrogen bond acidity, .beta.H.sub.2 is the hydrogen bond basicity, L.sup.16 is the gas-liquid partition coefficient of hexadecane and a, b, l, r and s are coefficients relating the solvation properties of the polymer to those of the vapor. The regression constant, c, is used to allow empirical fitting of the data. Coefficients for many polymeric materials have been determined using partition coefficients calculated from gas-liquid chromatographic retention data at high temperatures in inert atmospheres (Patrash and Zellers, 1993). It has been determined by McGill et al. (1994) that selectivity of polymeric sorbent layers can be optimized by evaluating ratios of LSER coefficients. Table III contains laser regression coefficients for fourteen polymers at 25.degree. C. For example, although each of the partition coefficients of fluoropolyol, 1-(4-hydroxy, 4-trifluoromethyl,5,5,5,-trifluoropentene and poly(4-vinylhexafluorocumyl alcohol) for dimethyl methylphosphonate a simulant for alkylphosphonate nerve agents, is relatively high, the relative magnitudes of the partition coefficients are arranged in the order of the ratio of acidity to basicity.
Needs exist generally for identifying industrial or polluting chemicals in liquid, gas or vapor forms. Needs also exist for detectors for chemical and biological warfare agents meeting a wide range of advanced specifications. Those specifications include reduction in size and weight of chemical agent detection instrumentation, development of chemical agent monitors that can be used with water supplies, integration of chemical and biological warfare detection instrumentation, development of miniature instrumentation with few or no moving parts and reduction in detector cost to provide distributed wide area networks for advanced warning capability. The defense against chemical and biological warfare agents involves detection of potential threats, development and utilization of protective equipment, development of vaccination and post-exposure prophylaxis measures and fabrication of structures with barriers to the toxic agents which are suitable for decontamination procedures. Threat identification is necessary prior to engagement, during battle and after battle during decontamination procedures. Additional uses of sensors for chemical warfare materials may be found in treaty verification, demilitarization, environmental monitoring and characterization of materials acting as barriers to agent diffusion.
Several methods are currently available for long-range threat identification using laser-based techniques such as light detection and ranging (LIDAR) and for laboratory analysis of chemical warfare agents using gas chromatography (miniCAMS), surface acoustic wave (SAW) technology or ion mobility sensor (IMS) technology, or for biological agent monitoring using light addressable potentiometric sensor (LAPS) or chemiluminescence approaches. There remains a need for lightweight, high-sensitivity sensors with rapid response times for use in threat identification, area monitoring, special forces operations, decontamination, demilitarization, treaty verification requirements and antiterrorism.